This proposal is for continuation of a program of research on the physiology and biophysics of visul photoreceptors, using arthropod eyes as the principal experimental material. More specifically, we are interested in the properties of visual pigments as revealed by microspectrophotometry; the organization of these pigments in the rhabdom and the physical properties of the rhabdomal membranes that provide the basis for polarized light analysis; and the cellular organization of the retina for color vision. Most arthropods have thermally stable metarhodopsins, and there is increasing evidence that regeneration of rhodopsin involves membrane replacement. We also intend to study the possible turnover of opsin using fluorescent probes. More evidence is needed on the role of intracellular pH, which is being approached by a photometric method. A pilot study of color discrimination in hummingbirds has also been undertaken.